1. Field of the Invention
The present invention relates to a lighting apparatus, especially for the automotive field, which comprises a light source, a lens having a diffractive structure on at least one of its surfaces and a diaphragm arranged between the light source and the lens. Furthermore the invention also relates to a lens of this type for the above-described lighting device and to a method of making this lens.
2. Description of the Related Art
A lighting apparatus is described in DE 42 15 584 A1, which has a light source, a reflector and a light disk. A diffraction grating is arranged between the light source and the light disk, through which the light reflected from the reflector and originating from light source travels. The diffraction grating has regions with micro-prisms as diffraction elements, through which light aimed in a certain direction is diffracted or deflected. The diffraction-optic elements are formed as scattering elements in other regions of the diffraction grating. These scattering elements distribute or scatter the light uniformly within a certain angular range. This sort of lighting device can, for example, be used as an auto headlight.
A headlight with a parabolic reflector based on these principles is described in DE 42 15 584 A1. It comprises a lamp, a parabolic reflector, a diaphragm and a scattering disk, which is built into the front disk.
A comparable auto signal light is also described in FR 2 785 364 A1. This light comprises a light source and a screen, which has the diffractive elements at least in one region, in order to produce a spread out light beam, whose angular distribution is continuous. The screen region is divided into blocks, which each have a diffractive structure. A very uniform light beam may be produced by division into individual blocks, which all produce the same light distribution.
A motor vehicle headlight for low beam or fog light is described in DE 35 07 013 A1. This headlight has a non-color correcting objective, which images the diaphragm edge as a light-dark boundary of the light beam on the road, and a projection element, which guides the rays of the light beam forming a color fringe at the light-dark boundary into its bright region. Since the light beam again mixes the different colors, the white light beam prescribed by law is produced without the troublesome color fringe. If a corrective element, which divides the light beam propagated from the objective into a differentiated light beam, i.e. into very many small light beams, is used, a white light-dark boundary is obtained with a slight coloration. A suitable corrective element has a cylindrical lens or an annular lens, which are formed as a collecting lens or a scattering lens with different indices of refraction and/or widths. These lenses require that the useful surfaces of the corrective elements must be only partially covered. The objective and the corrective element can be made from one piece. Pressing especially is used to make the part or side of the objective including the corrective element.
The so-called poly-ellipsoidal headlight (PES), which has a plane-aspheric lens instead of a light scattering disk, has been available since the mid-80s. In more recent times the lens has been lens is mated to a threefold ellipsoid reflector or also to a free-form reflecting surface.
The so-called poly-ellipsoidal headlight (PES) is described, for example, in WO 99/00623 A1. The motor vehicle headlight described there comprises a light source and a lens. The lens has a non-holographic surface structure, which acts as diffractive microstructure. Preferably refractive elements, i.e. lenses, are provided with a diffractive microstructure. The refractive-diffractive elements can be designed so that certain optical functions can be performed by the diffractive microstructure. These functions result in altogether thinner refractive elements without thickness variations. The most important optical function is based on legal requirements, which define or proscribe the direction in which the light beam is guided and the intensity distribution it must have over the surface. Additional parameters, which can be controlled by the design of the diffractive microstructure, are the uniformity, the color and light scattering properties. The facts that blue light is refracted most strongly and red light is diffracted most strongly are used for color correction. Since the design of the lens and the diffractive microstructure are harmonized with each other, the chromatic aberration of both elements can be compensated. The scattered light can also be compensated in the manner described according to WO 99/00623. Furthermore the diffractive microstructure can be used to provide the light from the signal lights with a predetermined pattern. For example the trademark of the automobile manufacturer can be made to appear in the brake lights or back lights. Also the color of the light beam can be adjusted by means of the diffractive microstructure. Thus one could conceive that different colors could be integrated in lights for different applications by means of diffractive microstructures.
Front headlights for low beam light generally produce a light distribution, which must be wider than it is higher. This is primarily produced by a threefold-ellipsoidal reflector with different half axes in the case of a poly-ellipsoidal headlight (PES). The upper half of the light beam is masked by a diaphragm and projected by a lens on the street. A sharp image of the diaphragm is thereby produced at a distance of about 10 meters. The transition between the illuminated region and the region blocked by the diaphragm is called the light-dark boundary. This light-dark boundary is adjustable so that it is clearly sharper with a poly-ellipsoidal headlight (PES) than with a headlight with a paraboloid reflector. In the standard headlight test the light distribution of a headlight on a measuring wall at a distance of 25 meters from the headlight is divided into a number of different zones and a plurality of test points. A significant parameter is the light intensity at the so-called HV point, which is about 25 cm above the light-dark boundary in the center of the light beam. There the light intensity may not exceed a defined value. The HV point is in the dark region of the light intensity distribution. When the image is not focussed or sharp the light-dark boundary spreads out and the light intensity at the HV point exceeds the permitted value. The light intensity at the HV point is thus also a measure of the scattered light. Up to now it has proven to be very difficult to keep the light intensity at the HV point below the permitted limiting value while maintaining all the remaining test standards.